1. Field of the Invention
The present invention relates to a process for improving the coefficient of traction and a traction drive fluid for use therein. More particularly, it is concerned with a process for improving the coefficient of traction at high temperatures, and a traction drive fluid for use therein, which has a good flowability at low temperatures, and a high viscosity index, and exhibits an excellent traction performance over a wide temperature range of from low temperature to high temperature.
2. Background Information
A traction drive fluid is a fluid to be used in traction drives (friction driving equipment utilizing rolling contact), such as continuously variable transmission for cars or industrial machines and hydraulic machines. In general, such traction drive fluids are required to have a high traction coefficient, a high stability against heat and oxidation and, furthermore, to be inexpensive.
In recent years, investigations have been made to reduce the size and weight of the traction drive unit, particularly for use in cars. With this miniaturization, the traction drive fluid to be used in such units has been required to have a performance high enough to be used under severe conditions, particularly to have a high traction coefficient, a suitable viscosity, and a high stability against heat and oxidation constantly over a wide temperature range of from low temperatures to high temperatures, specifically from -30.degree. to 140.degree. C.
The size of traction drive units is said to be inversely proportional to 0.45 powers of the traction coefficient of the traction drive fluid to be used (Technical Literature IC/FP-28R by Monsanto Company). According to this concept, the higher is the minimum traction coefficient in the temperature range in which said fluid is used, the more the miniaturization of traction drive units can be attained.
Various traction drive fluids have been proposed as in Japanese Patent Publication Nos. 338/1971 and 339/1971. These traction drive fluids, however, have failed to satisfy the abovementioned requirements, and have been involved in many problems. For example, compounds having a high traction coefficient at high temperatures produce a large agitation loss because of its poor flowability at low temperatures, and therefore, the transmission efficiency is low and the start-up property in low temperature is not sufficient. On the other hand, compounds which are of low viscosity and are excellent in transmission efficiency have a low traction coefficient at high temperatures, and as the temperature rises, their viscosities drop excessively, causing troubles in lubrication of the traction drive unit.
Furthermore, various ester compounds are disclosed in Japanese Patent Publication No. 44918/1986 as a traction drive fluid having a norbornane ring structure, but these ester compounds cannot withstand practical use, since they are very low in traction coefficients at high temperatures, and are lacking in thermal stability.
hydrogenated product of a dimer of camphene disclosed in Japanese Patent Publication No. 198693/1989 has a fairly high traction coefficient at high temperatures, but has a defect in flowability at low temperatures since it has such a high pour point as -27.5.degree. C.
When used as a traction drive fluid, fluids having small viscosity indexes rise largely in viscosity at low temperatures, and accordingly the agitation resistance of the fluid increases causing poor start-up properties at low temperatures of a CVT (continuously variable transmission). On the other hand, at high temperatures, the viscosity of the fluid drops too suddenly to retain an appropriate lubricating oil film, which causes fatigue and damage in the traction drive units. Consequently the viscosity index of the fluid is desired to be as high as possible.